Oxidation of methionines by oxochromium(V) cations: A kinetic and spectral study

The oxidation of methionine (Met) plays an important role during biological conditions of oxidative stress as well as for protein stability. By choosing [oxo(salen)chromium(V)] ions, [(salen)CrVO]+ (where salen = bis(salicylidene)ethylenediamine) as suitable biomimics for the peptide complexes that are formed during the reduction of Cr(VI) with biological reductants, the oxidation of methionine and substituted methionines with five [oxo(salen)chromium(V)] complexes in aqueous acetonitrile has been investigated by spectrophotometric, electron paramagnetic resonance (EPR) spectroscopy and electrospray ionization mass spectrometry (ESI-MS) methods. In aqueous solution [(salen)CrVO]+ ion is short lived, ligation of H2O to the Cr center takes place and [OCrV(salen)–H2O]+ adduct is the active oxidant. The reaction is found to be first order each in the oxidant and the substrate. The presence of water in the reaction system accelerates the reaction rate and an inactive, stable μ-oxo dimer is also formed during the course of the reaction. On the basis of spectral, kinetic and product analysis study a mechanism involving direct oxygen transfer from [OCrV(salen)–H2O]+ to methionine has been proposed as a suitable mechanism for the reaction.